The present invention relates to a charge pump circuit for generating a voltage by charging and discharging capacitors.
A flash memory incorporated in a microcomputer chip uses a high voltage during operation and therefore includes a charge pump circuit to generate a boosted voltage. The operation of the charge pump circuit requires a large current consumption from an external power supply, and electro-magnetic interference (EMI) noise caused by the current consumption is radiated to the outside of the microcomputer chip.
A high level of radiated EMI noise causes interference with devices around the microcomputer chip. Particularly, as for an IC (Integrated Circuit) used in an audio device or the like, caution needs to be exercised so that the frequency band of EMI noise radiated from the microcomputer chip does not overlap with the received frequency band of the audio device. Multiples of the operation clock frequency of the charge pump circuit are dominant in frequency components of EMI noise caused by the operation of the charge pump circuit. Accordingly, the operation clock frequency of the charge pump circuit is set in consideration of a frequency band to be restrained in an environment where the IC is used.
In a power supply circuit disclosed in Japanese Unexamined Patent Publication No. 2005-20971, once a comparator detects an excess of a reference voltage, the comparator output is not switched thereafter within one pulse of a clock signal. This prevents a charge pump from operating at high frequencies and prevents the occurrence of noise.